Many camshaft phase shifting mechanisms (herein also termed phasers) have been proposed, and are in use, which are actuated hydraulically, often using engine oil. While these phasers have been demonstrated to be effective, they can have a high oil demand, sometimes requiring an auxiliary pump and an associated cost penalty. The presence of large amounts of oil in the engine cylinder head can cause problems with oil drainage and aeration, together with the problems associated with belt contamination in the case of belt driven camshafts.
Furthermore, the use of hydraulic actuation requires a control valve, which can control the oil flow in response to electrical inputs from the engine ECU, and this can have significant cost and packaging implications.
U.S. Pat. No. 5,056,477 discloses a phase shifting mechanism for an engine camshaft that is subjected to torque fluctuations during operation. The mechanism comprises a drive member and a driven member coupled for rotation with one another by means of a closed hydraulic circuit formed of a plurality of variable volume working chambers that are connected to one another in such a manner that the volume of one increases as the volume of another decreases. Flow of fluid between the working chambers, which is controlled by a solenoid valve, causes the phase of the drive member to be shifted relative to the driven member.